High pressure tube hydroforming is a multiphase forming technique that forms a tube-shaped workpiece into a desired shape by placing the workpiece in dies having a shaping void of a desired shape and injecting fluid, for example, water, into the tube under high pressure, instead of separately machining workpieces with various press dies and then welding the machined workpieces to each other, when forming complex components. The hydroforming is a tube forming technique that has a high material recovery degree and high productivity.
Generally, typical double/multilayered tubes are fabricated by filling adhesives or synthetic resin in a space between inner and outer tubes to couple the inner and outer tubes to each other or by heating or cooling the inner or outer tube for heat shrink fitting.
However, such tubes have problems as follows.
First, for the double/multilayered tubes produced by coupling the inner tube and the outer tube using the adhesives, a bonding force between the inner and outer tubes can be weakened due to chemical changes of the adhesives resulting from variation of surrounding temperatures or conditions.
Second, application of the adhesives or filling of the synthetic resin requires complicated processes, such as surface treatment, solidification, and the like, thereby reducing productivity.
Third, the adhesives or the synthetic resin are not decomposed even after extended periods of time and cause an increase in manufacturing costs as well.
Fourth, the heat treatment for heat shrink fitting requires expensive heat treatment equipment and cannot achieve uniform coupling over the surfaces of the tubes when the tubes have an elongated shape.